Method for growing high-melting-point single crystals and an apparatus therefor



Dec. 17, 1968 KEN|H| SHlRQKl ET AL 3,416,898

METHOD FOR GROWING HIGH-MELTING-POINT SINGLE CRYSTALS AN v AN APPARATUSTHEREFOR Filed July 27, 1967 Mo TOR uN/r /l7 INVENTORS KEN/CHI SHROK/SEI/CHI SAlfO A 7' TOR/VE X5.

United States Patent Oioce 3,416,898 Patented Dec. 17, 1968 3,416,898METHOD FOR GROWING HIGH-MELTING- POINT SINGLE CRYSTALS AND AN AP-PARATUS THEREFOR Kenichi Shiroki and Seiichi Saito, Tokyo, Japan,assignors to Nippon Electric Company, Limited, Minato-ku, Tokyo, Japan,a corporation of Japan Filed July 27, 1967, Ser. No. 656,582 Claimspriority, application Japan, July 30, 1966, 41/ 49,964 Claims. (Cl.23-301) ABSTRACT OF THE DISCLOSURE A rod of high-melting-pointsubstances is supported from one end thereof in the direction of itsaxis with the other end exposed for melting. A hollow gas flame isdisposed coaxially relative to the rod, whereby the end of thel llamecontacts and melts the exposed end, following which a single crystal isdrawn from the melted end, the single crystal being within andsurrounded by the hollow ame during the drawing.

A burner means comprised of tconcentrically arranged inner and outerannular nozzles is coaxially arranged relative to the rod supporting therod. Located coaxially between the rod-support means and the burner is aseed crystal suport means, the rod-support means and the crystal supportmeans being adapted to move along 4their common axis.

DETAILED DESCRIPTION OF THIS INVENTION This invention relates generallyto a method for growth of single crystals of high melting pointsubstances and an apparatus therefor and more particularly to a methodfor insulating single crystals comprised substantially ofhigh-melting-point oxides.

The ame fusion technique known as the Verneuil method has been adopted,ypredominantly over other crystal growth processes for the growth ofhigh-melting- -point single crystals, -notably for the preparation ofsingle crystals of oxides. This technique consists generally of aninverted oxhydrogen burner which opens into a muler and a support rod ofrefractory material located centrally in the mulle, which can -belowered as the boule grows. As oxygen, for example, carries tineparticles of the raw material of which a single crystal is to be madeinto the intense heat of a burner flame, they fuse and fall by gravity-on the molten upper surface of the boule so that boule growth may takeplace. When using a crucible for changing a raw material, the cruciblematerial will invariably diiuse into the molten raw material. The amefusion method is inherently free from this possibility, since nocrucible is used. This advantage of the flame fusion technique is offsetby the following disadvantages: In the process of 'boule growth, rapidlowering in temperature occurs at that layer of the -boule just beneaththe molten upper surface in succession `which, in turn, developsinternal thermal strains in that layer to become the cause ofintroducing crystallagrophic imperfections throughout the grown boulesuch as residual strain, dislocation, etc. To eliminate theseimperfections, annealing of the grown crystals at high temperatures hasbeen necessary.

Aside from this flame fusion technique, another technique has been usedfor growing single crystals, such as of a semiconductor, which iscommonly called the Czochralski or pulling method. According to thismethod, the raw material is heated and melted in a Crucible and a seedcrystal is lowered into the surface of the melt and then slowlywithdrawn so that the melt may cling onto the seed crystal in the formof a single crystal. An advantage of the pulling method over theVerneuil is that `since the growing crystal is continuously heated byheat energy of the melt in the orucible, the temperature gradient in thegrowing crystal is maintained less than that encountered when using theVerneuil method, with the result that a whole single crystal with lessimperfections, such as residual strain or dislocation, can be grown. Adrawback of this method is the tendency of the melt contaminated by thecrucible material, of intermingling of impurities into the raw material,and eventually of degrading the quality of grown single crystals.Another drawback of the pulling method is the following: Although acarbon crucible, if used, is heat-resistant to high temperatures inexces of 2,000 degrees centigrade is vacuum or in a reducing gasatmosphere, it is oxidized quickly at temperatures above 500 degreescentigrade in an oxidized atmosphere. To obviate this inconvenience,suppose that a crucible made of platinum is used. While a platinumcrucible may be used at temperature above 1,500 degrees centigrade inair, it can scarcely be used at such high temperatures in a hydrogenatmosphere, because platinum is chemically attacked by the hydrogen.Such a problem imposes a limitation on both the kind of atmosphere to beemployed and the usable orucible material lfor the growth of singlecrystals.

Accordingly, the principal object of the present invention is to providea new and improved single-crystal growing for high-melting-pointsubstances that would enable strain-free, dislocation-free, andcontainimationlfree single crystals to be grown.

Another object of this invention is to provide a new, improved, andstraightforward crystal growing instrument and method to replace any oneof the conventional crystal growth methods with respect to simplicity ofraw material preparation, dispensability of a high-melting-pointcrucible, and in improved yields.

The se and other yobjects will more clearly appear when ta'ken inconjunction with the following description and the accompanying drawingwhich illustrates diagrammatical a cross-sectional View in elevation(not to actual proportions) of the furnace apparatus employed to eifectgrowth of single crystals in accordance with the invention.

An embodiment of the crystal growing method according to the principlesof this invention is as follows:

The free end of a rod made of molded raw material powder is melted :bythe intense heat of a ilame such Ias from an oxyhydrogen burner. A seedcrystal, for instance, is lowered in the direction approximately alongthe axial line of the burner so that the seed crystal is caused to dipinto the molten upper surface of the rod. It is then withdrawn upwardgradually at a constant rate so that the melt may cling onto the seed asa single crystal.

It will be readily recognized by one skilled in the art that the presentcrystal growing method is advantageous over the conventional techniquesin the following respects:

No Crucible being needed, there is no possibility of contamination ofthe growing single crystals kby :a crucible material.

Since growth of single crystals takes place by raising the growingcrystal away from the molten surface of the rod, any suitable atmosphereunder which crystal growth takes place can be selected in advance. Forexample, in case of an oxyhydrogen burner, either a hydrogen or anoxygen gas may be selected as the atmosphere surrounding directly thegrowing crystal.

Substantially strain-free single crystals can be grown, especially whenthe diameter of the rod made of molded raw material powder, or moreparticularly, the diameter of the molten zone is designed to besuiciently larger than that of the growing crystal, because this designpermits the temperature gradient in the growing crystal to be smallenough.

The apparatus adapted for carrying the method of this invention intoeffect comprises a special burner installed on one end of a furnace.This burner has at least two concentric annular nozzles. Each of thenozzles has one annular opening or a plurality of openings symmetricallyarranged about the central axis of the nozzles. Any one of an inflamablegas and a gas for supporting combustion of the inflamable gas is fedAfrom one nozzle, while the other of the gases is fed from the othernozzle. The flow of the gas fed from the outer nozzle should bestoichiometrically much more than the flow of the gas from the innernozzle, so that a flame formed through the combustion within the furnacehas a highest temperature portion of a hollow cylindrical form enclosingan inner ame portion. At the other end of the furnace there is provideda support for `a rod of the high-melting-point substance. This supportis moved by a driving means in the direction of the central axis of thenozzles, so that the end portion of the rod is kept in the highesttemperature portion of the llame. Into the inner llame portion a singlecrystal seed of the `substance is introduced lby another support. Thissupport is also moved in the direction of the central axis of thenozzles so as to bring at rst the seed into contact with the melting endsurface of the rod and then draw up the seed apart from the melting endsurface. Hence, the growing single crystal is surrounded by the innerflame portion according as the seed is drawn up.

The above-mentioned and other features of this invention, will be moreclearly understood by the detailed description given of a most preferredembodiment of the single crystal growing method according to thisinvention in conjunction with the accompanying figure.

Referring to the drawing, it will be understood that a rod 4 made ofmolded alumina powder is clamped on a support 3 connected to a shaft 2which is driven by a motor unit 1 through a gear mechanism (not shown)so as to go up and down, the major part of the alumina molded rod 4being installed in a furnace 7. The furnace 7 has a hollow cylindricalspace which is surrounded by a muffle 6 of refractory material equippedwith an inspection window of transparent heat-resistant material and anexhaust pipe 22 for connecting the inside of the furnace 7 with theoutside. The top of the furnace is closed by the tip of a burner 9, thelbottom being closed by a base lid 23. The burner 9 has an inner and anouter concentric annular nozzle 15 and 16 from which an oxyhydrogenllame 8 is fed into the hollow space. The burner 9 can feed oxygen andhydrogen gases supplied thereto through two inlets 10 and 11 to thefurnace 7 through the two annular chambers 12 `and 13, the nozzles 15and 16, and annular openings 14 and 14 being symmetrically arrangedabout the central axis of the burner 9, respectively. The ow of thehydrogen gas fed from the outer nozzle 16 is stoichiometrically muchmore than that of the oxygen gas fed from the inner nozzle 15 so as toform a flame whose highest temperature portion 8 is of a hollowcylindrical form. The excess hydrogen gas in the furnace 7 flows to theoutside through the exhaust pipe 22 and burns. The burner 9 may have athird outermost annular concentric nozzle (not shown) or leave anannular gap between the top inner circumference of the mule 6 througheither of which oxygen or another gas for supporting combustion of thehydrogen gas fed from the outer nozzle 16 is supplied in order to burnthe excessive hydrogen gas. In case the hydrogen gas and thestoichiometrically less amount of oxygen gas are fed Afrom the inner andthe outer nozzles 15 and 16, respectively, the excess oxygen gas ows tothe outside through the exhaust pipe 22. In this case both the exhaustpipe 22 and the base lid 23 may be dispensed with.

Another shaft 19 which is movable in the axial direction and rotatableby a motor unit 17 is installed through the central portion of theburner 9 in alignment with the shaft 2. At the bottom of the shaft 19,there is a clamp for holding a seed crystal 18. At first, the seedcrystal mounted at the lbottom of the shaft 19 is lowered so as to dipinto the central part of the molten top end surface 20 of the aluminarod 4. With the continuous attention of an operator who observes thefurnace interior through the inspection window 5, the rod is graduallyraised at a constant rate. Then the melt clings onto the seed bottom inthe form of a single crystal. As the growing of the single crystal 21proceeds, the molten top 20 of the rod is gradually dissipated and thetop surface of the rod is lowered. To compensate lfor the lost amount,the rod 4 should be gradually raised by means of the shaft driving unit1.

Uniform growth of a single crystal of aluminia took place with ourexperiment by performing this process continuously until most of the rawmaterial had been exhausted.

It will be seen in the present embodiment that an oxygen gas isintroduced inside of the highest temperature portion 8 -constituting anoxygen-hydrogen boundary in an oxyhydrogen flame so that a singlecrystal of alumina may grow in the oxygen atmosphere.

As a result of conducting single crystal growth for :artificial rubies,using this furnace apparatus at the oxygen and hydrogen flow rates,respectively, of l5 litres and 37 litres per minute, a pulling speed ofmicrons per minute, and a central flame temperature of 2,100 degreescentigrade, it was discovered that substantially strain-free andcontamination-free ruby single crystals could be produced, without theneed of a subsequent annealing process at high temperature for theprevention of fractures as had commonly occurred when using the Verneuilmethod.

It will ybe understood that the burner construction such as gas passagesfor the above-mentioned gases may be suitably modified depending on thequality of a single crystal to be grown.

For instance, the passages for hydrogen and oxygen gases in theabove-mentioned embodiment may be interchanged in the illustration tomake a hydrogen gas as the atmosphere for surrounding a `growing singlecrystal of manganese monoxide (MnO), for example, thereby to preventexcessive oxidation. Further, the kinds of gas to be used should by nomeans be restricted to an oxyhydrogen gas; any suitable combination of acombustible and an oxidizing or inert gas may lbe used, such as propane,town gas, each mixed with air, or an oxyhydrogen gas mixed with argon.Flames formed by the combustion of these gases may be circular in crosssection as in the illustration or may be any suitable geometricalconfiguration whenever such necessity arises.

It has 'been mentioned in conjunction with the embodiment that the seedcrystal is to be pulled upward, but it will be seen that it may bepulled downward in case of a furnace construction which is exactly theupside-down of the illustration because no gravitational effect isutilized in the present crystal growing method.

It will be obvious `by one skilled in the lart that the present methodcan find application not only in the growth of alumina single crystals,but also in that of single crystals of high-melting-point metals such asplatinum, rhodium, or iridium, provided that the kinds of flame (such aschemical combustion ames for oxides and plasma flames for metals) flowrates of the gases, flame temperature, the pulling rate, and others besuitably controlled or selected according to the kind of substance to begrown.

We claim:

1. A method of growing a single crystal of a highmelting-point substancefrom a rod of said substance which comprises: supporting a rod of ahigh-melting-point substance from one end thereof in the direction ofits axis; forming a hollow ame of an inflammable gas by combustion withanother gas and disposing said hollow ame so that it is alignedcoaxially with said rod whereby the end of said hollow flame contacts anend of said rod and melts it; and then drawing a single crystal from themelted end of said rod along the axis thereof and the axis of saidhollow llame, the single crystal being completely surrounded by saidhollow flame and spaced therefrom during the growing thereof.

2. The method of claim 1, wherein said hollow flame is substantiallycylindrical in shape, and wherein said hollow ame is formed by feedingthe inflammable and combustion gases through concentrically arrangedouter and inner annular nozzles respectively, the intlammable gas beingfed through the outer annular nozzle at a rate stoichiometricallygreater than the gas flowing through the inner annular nozzle.

3. The method of claim 2, wherein the single crystal is drawn from themelted end of said rod Iby contacting said melted end with a seedcrystal and then moving said seed crystal axially from said melted endwithin said hollow llame whereby to grow said single crystal.

4. An apparatus for growing a single crystal of a highmelting-pointsubstance from a rod of said substance which comprises: a rod-supportmeans for supporting a rod of high-melting-point substance from, one endthereof in the direction of its axis, the other end of the rod beingexposed for heating; means for moving said rod-support means along therod axis; burner means comprising concentrically arranged inner andouter annular nozzles coaxially disposed relative to said rod-supportmeans for heating and melting the exposed end of said rod; one of saidannular nozzles being adapted to feed an inllammable gas, the othernozzle a gas for combusting said inflammable gas, whereby t0 form ahollow annular flame coaxially with the rod axis, means extendingcentrally of and withdrawable through said nozzles for supporting a seedcrystal coaxially and centrally of said nozzles and said rod-supportmeans; and means for moving said seed crystal-support means to and fromsaid exposed rod end along the axis of said rod, whereby to grow asingle crystal from the melted end of said rod.

5. The apparatus of claim 4, including a furnace comprising a muiTlewith a chamber therein coaxial with the rod-support means and theburner, wherein the rodsupport means enters the chamber from one end,wherein the concentrically arranged annular burner is mounted againstthe other end of the chamber, and wherein the means for supporting theseed crystal is coaxially arranged to move along the axis of saidcham-ber between the rodsupport means and the burner means.

References Cited UNITED STATES PATENTS 3,190,728 6/1965 Vanderink 23-273FOREIGN PATENTS 243,201 11/ 1925 Great Britain.

NORMAN YUDKOFF, Primary Examiner.

GENE P. HINES, Assistant Examiner.

